As explained in https://github.com/zopefoundation/ZEO/issues/209 there is possibility of data
corruption when ZEO5 client loads data from ZEO4 server.

A fix for this is not trivial and would have to forward-port
load-tracking in client from ZEO4 to ZEO5. However, as discussed in
https://github.com/zopefoundation/ZEO/issues/209, we believe that noone
is actually using ZEO5.client-ZEO4.server configuration. Thus, given
that it was already planned to drop ZEO4 support soon, it was decided to
drop support for ZEO4 server instead of fixing it.

This contains 3 patches:

- patch 1 adds test that catches mentioned data corruption problem
- patch 2 corrects documentation for credentials (ZEO5 feature) and user/password/realm (ZEO4-only basic auth)
- patch 3 actually removes support for interoperability with ZEO4.

Please see individual patches and their descriptions for details.

In particular I believe the following excerpt from patch 3 is important to note here as well:

---- 8< ----
- we do _not_ remove verify_invalidation_queue added by Jim in 2016 via
  5ba506e7 (Fixed a bug handling ZEO4 invalidations during cache
  verification) with the following message:

        ZEO4 servers can send invalidations out of order with
        ``getInvalidations`` results, presumably because ``getInvalidations``
        didn't get the commit lock.  ZEO4 clients worked around this (maybe
        not directly) by queuing invalidations during cache verification.
        ZEO5 servers don't send invalidations out of order with
        ``getInvalidations`` calls and the ZEO5 client didn't need an
        invalidation queue, except they do need one to work correctly with
        ZEO4 servers. :/

  This feature, even-though it is commented as being ZEO4-only, looks
  too risky to be removed in stable branch, especially taking into
  account that in https://github.com/zopefoundation/ZEO/pull/195
  @d-maurer instead of removing, preserved this queue in a similar form:

  https://github.com/zopefoundation/ZEO/blob/30e271bbe5380a1fe65f3ca776bc70b4b29b58d9/src/ZEO/asyncio/client.py#L90
  https://github.com/zopefoundation/ZEO/blob/30e271bbe5380a1fe65f3ca776bc70b4b29b58d9/src/ZEO/asyncio/client.py#L277-L280
  https://github.com/zopefoundation/ZEO/blob/30e271bbe5380a1fe65f3ca776bc70b4b29b58d9/src/ZEO/asyncio/client.py#L630-L641

  I believe it is better be safe than sorry.
---- 8< ----

Thanks beforehand,
Kirill

/reviewed-by @dataflake, @d-maurer
/reviewed-on https://github.com/zopefoundation/ZEO/pull/213